The biggest time difference on land is between China and Afghanistan, 3 and a half hours, but there is no border crossing point there: https://en.wikipedia.org/wiki/Wakhjir_Pass

AFAIK The biggest time difference where you can cross is between Tajikistan and China, you would have to set your clock by 3 hours. There is one border crossing there Kulma Pass

Kiribati switched to the other side of the date line in 1994, so there are 3 different days at the same time on the Globe for an hour:

• Kiritimati, Kiribati, UTC+14: Sunday, 00:30
• London, UK, UTC+1: Saturday, 11:30
• Midway Island, US, UTC-11: Friday, 23:30

https://www.timeanddate.com/worldclock/converter.html?iso=20231014T103000&p1=274&p2=136&p3=1890

So be careful when you ask what day it is